Vehicles such as mining vehicles typically require spring-applied, hydraulically-released parking brakes. Springs are used to clamp the brake linings together and when hydraulic pressure is applied, one or more springs are compressed and the brake is released. Conversely, when hydraulic pressure is released, the one or more springs are decompressed or expanded, and the brake is engaged.
The source of the hydraulic pressure for releasing and engaging a brake is generally from the hydraulics of the vehicle since most mining vehicles include hydraulic systems. However, on small vehicles like the Toyota® Land Cruiser®, there is no vehicle hydraulic system. In these cases, a pump pack including an electric motor, a hydraulic pump, a hydraulic reservoir, and a solenoid valve is used.
The electric motor turns a hydraulic pump, which is used to create pressure to release the brakes. The solenoid valve is used to hold the pressure in the line keeping the brake released, so that the pump does not have to run continuously. The solenoid is a normally open device. This means that in absence of electrical power, the solenoid will open the brake line to tank, which will cause the pressure to fall to zero.
Thus, to engage the brakes, power to the solenoid and motor is turned off, pressure then falls, and the brakes engage. To release the brakes, power to the solenoid and motor is applied, which closes the solenoid, turns on the motor, and allows brake line pressure to build, so that the brakes release. Prior art pump packs use electrical relays and a pressure switch, which are mechanical switches, to signal the motor to start and stop, depending upon pressure. These pump packs also use relays to control power to the solenoid.
As it is critical that the brakes of these vehicles are engaged under certain conditions, these prior art pump packs generally include three independent conditions that will engage the brakes.
First, an engine oil pressure switch is used to signal relays on the pump kit to interrupt power to the solenoid and motor, which drops pressure and engages the brakes. If the engine is shut off, the brakes come on. Second, door switches are generally wired to the pump kit, so that if the door is opened, associated relays trip, and the brakes engage. Third, these pump packs generally include an emergency stop button on the dash panel that the operator can press to trip associated relays and cause the brakes to engage. By pulling the button, or by twisting the button in certain styles, an operator can cause a solenoid to close and the motor to run in order to release the brakes.
As seen from the above, hydraulic pressure is needed to release these brakes. Thus, a problem arises when a vehicle of this nature breaks down. Since the engine oil pressure switch trips the relays to release pressure and turn off the motor to engage the brake, the pump will not run if the engine is not running. This is most problematic in a situation where the engine has broken down since the brakes will not release. Further adding to the dilemma, the vehicle cannot be towed because the brakes are locked up. This requires an operator to create pressure for the hydraulic system to release the brakes.
The prior art solutions to this problem are inconvenient and potentially hazardous. One such solution is to run jumper wires across relays on the pump kit, so that the pump will operate regardless of whether or not the engine is running. This requires that the operator leaves his seat and go to the pump kit, which is mounted either behind the seat, under the hood, or in the bed of the truck. The operator then has to determine which relays to bypass with jumper wires, and he may have to make jumper wires from raw wire.
If the operator can do this, the jumper wires bypass the relays, which causes the pump to run, which releases the brakes while he is out of the vehicle. This can result in the brakes being released while he is standing next to the vehicle, under the vehicle, or in the bed of the vehicle. In all cases, the brakes release while he is somewhere other than in his seat, buckled in, and in control. Obviously, the vehicle can roll while he does this operation, thereby creating a potential hazard. This is especially true in mining vehicles, which can operate on steep grades underground.
Moreover, many mines have volatile gases, which can be explosive when exposed to sparks. Running bypass wires across mechanical relays creates potential for sparking. The pumps also use petroleum-based fluids, which are flammable and should not be exposed to sparks.
Another hazard in the use of jumper wires is that bypassing relays can, in some cases, disable the emergency brake. Depending upon which relays are bypassed, the operator may or may not be able to engage the parking brakes in an emergency should the primary brakes fail while he is being towed. The operator would have to have a wiring diagram and the necessary technical expertise to understand the circuits to prevent disabling the brakes altogether.
An additional hazard could occur once the vehicle is repaired. Since the relays were bypassed by a jumper wire, someone has to remember to remove the jumper wire once the vehicle is fixed. If they fail to do so, they will be operating the vehicle without a proper parking and emergency brake and would lack awareness of this hazard.
Another prior art solution to releasing the brake is to use a hydraulic hand pump. Of course, this requires that the operator has a hand pump available to him. In this scenario, the operator must attach the hand pump to a brake line, pump up the pressure to release the brakes, and then lock it off for towing. Again, the driver is not in his seat and in control of the vehicle while he is doing this operation. Moreover, just as described above, someone has to remember to remove that hand pump once the vehicle is repaired, or the vehicle may be operated with no available parking and emergency brake.
Thus, a need exists in the art for a parking brake system having an improved mechanism for releasing the brake.